Anti-islanding protection systems for synchronous machine based distributed generators

ABSTRACT

A system for providing anti-islanding protection of a synchronous machine based distributed generator includes a frequency sensor configured to generate a generator frequency signal, a bandpass filter configured for filtering the generator frequency signal, a governor controller configured for using the filtered frequency signal to generate a power feedback signal, and a governor summation element configured for summing the filtered frequency signal, the power feedback signal, and a reference power to provide an electrical torque signal. Another system includes a voltage sensor configured to generate a generator terminal voltage signal, a feedback power calculator configured for generating a reactive feedback power signal, a bandpass filter configured for filtering the terminal voltage signal, and a PI controller summation element configured for summing the filtered terminal voltage signal, the reactive power feedback signal, and a reference reactive power to provide an error signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Division of U.S. patent application Ser. No.10/966,665 entitled “Anti-Islanding Protection Systems for SynchronousMachine Based Distributed Generators” filed Oct. 15, 2004, now U.S. Pat.No. 7,202,638 which is herein incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

This invention was made with Government support under NREL contractnumber NAD-1-30605-01 and DOE contract number DE-AC36-99GO10337. TheGovernment has certain rights in the invention.

BACKGROUND

The invention relates generally to systems for anti-islanding protectionof a synchronous machine based distributed generator from a feederhaving been disconnected from an electrical grid.

The distribution of electric power from utility companies to customersutilizes a network of utility lines connected in a grid-like fashion,referred to as an electrical grid. The electrical grid includesindependent energy sources energizing the grid in addition to utilitycompanies energizing the grid, with each independent energy source beingreferred to as a distributed generator (DG). A typical DG includes sometype of power conditioner or converter, such as an inverter for example,that feeds power to the feeder system of the grid. Exemplary DGs includebut are not limited to energy storage devices (such as batteries orflywheels, for example), photovoltaics, micro-turbines, fuel cells,engine-generator sets, and wind-turbine-generator sets. A conventionalfeeder system typically includes distribution lines that provide powerfrom the grid or DG to a customer load via electrical disconnects anddistribution transformers. Even with the presence of a DG connected tothe grid, the utility company is still the main source of power and inmany cases controls the system voltage and frequency within nominalvalues.

Under certain conditions, the utility power source may be disconnectedfrom the grid and feeder system, leaving the DG directly tied to theload or disjointed grid branch, which is referred to as islanding. Theisolated section of the grid being powered by the DG is referred to asan island. Unintentional islanding results in a situation where thevoltages and frequencies on the disjointed grid branch are outside ofthe direct control of the utility company because that branch isprimarily energized by one or more DGs. Accordingly, monitoring anddisconnect schemes, referred to as anti-islanding schemes, are used totimely disconnect a DG from the feeder in the event that grid power froma utility company has been disconnected from the feeder.

Anti-islanding schemes presently used or proposed include passiveschemes and active schemes. Passive schemes are based on localmonitoring of the grid signals, such as under or over voltage, under orover frequency, rate of change of frequency, phase jump, or systemharmonics, for example. Active schemes are based on active signalinjection with monitoring of the resulting grid signals, such asimpedance measurement for example, or active signal injection withactive controls, such as active frequency shifting or active voltageshifting for example. With passive schemes, close power matching betweenthe DG output and the total load may result in a sustained island due tothe voltage and frequency holding within nominal ranges. With activeschemes, some distortion may occur in the output current waveform,thereby resulting in a tradeoff between islanding detection time andwaveform distortion, with faster detection typically resulting in highertotal harmonic distortion (THD). Accordingly, there is a need in the artfor an anti-islanding arrangement that reduces these drawbacks.

BRIEF DESCRIPTION

Briefly, in accordance with one embodiment of the present invention, asystem for providing anti-islanding protection of a distributedgenerator with respect to a feeder connected to an electrical gridcomprises: a frequency sensor configured to generate a generatorfrequency signal representative of a rotational speed of the generator,a bandpass filter configured for filtering the generator frequencysignal, a governor controller configured for using the filteredfrequency signal to generate a power feedback signal, and a governorsummation element configured for summing the filtered frequency signal,the power feedback signal, and a reference power to provide anelectrical torque signal for use by a prime mover coupled to thegenerator.

In accordance with another embodiment of the present invention, a systemfor providing anti-islanding protection of a distributed generator withrespect to a feeder connected to an electrical grid comprises a voltagesensor configured to generate a terminal voltage signal representativeof a terminal voltage of the generator, a feedback power calculatorconfigured for generating a reactive feedback power signal, a bandpassfilter configured for filtering the terminal voltage signal, and aproportional-integral (PI) controller summation element configured forsumming the filtered terminal voltage signal, the reactive powerfeedback signal, and a reference reactive power to provide an errorsignal for use by a proportional integral controller in generating avoltage reference signal (V_(ref)) for the generator.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a block diagram of an active power anti-islanding protectionsystem in accordance with one embodiment of the present invention.

FIG. 2 is a block diagram of a reactive power anti-islanding protectionsystem in accordance with another embodiment of the present invention.

FIG. 3 is a block diagram of a combined active and reactive poweranti-islanding protection system in accordance with another embodimentof the present invention.

FIG. 4 is a graph illustrating terminal voltage, frequency, and rate ofchange of frequency without active anti-islanding control.

FIG. 5 is a graph illustrating terminal voltage, frequency, and rate ofchange of frequency with active power anti-islanding control.

FIG. 6 is a graph illustrating terminal voltage, frequency, and rate ofchange of frequency with reactive power anti-islanding control.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of an active power anti-islanding protectionsystem 10 in accordance with one embodiment of the present inventionwherein system 10 includes a frequency sensor 16 configured to generatea generator frequency signal ω_(m) representative of a rotational speedof a generator 12, a bandpass filter 22 configured for filtering thegenerator frequency signal, a governor controller 20 configured forusing the filtered frequency signal to generate a power feedback signalP_(fb), and a governor summation element 24 configured for summing thefiltered frequency signal, the power feedback signal, and a referencepower P_(ref) to provide an electrical torque signal T_(e) for use by aprime mover 26 coupled to generator 12. Typically frequency sensor 16obtains electrical or mechanical measurements.

In a more specific embodiment, anti-islanding protection system 10further includes a grid controller 28 configured for receiving thegenerator frequency signal and generating a grid disconnection signal(GC_(d)) upon the generator frequency signal becoming out of a nominalfrequency range (typically, but not always, due to the opening ofutility disconnect device 50). The grid disconnection signal can be usedto control generator disconnect device 46 so as to disconnect generator12 and local load 44 from external load 48 and grid 14, for example.Each of Local load 44 and external load 48 may comprise a passive oractive load, for example.

In a still more specific embodiment, anti-islanding protection system 10includes a frequency difference element 18 configured for obtaining adifference ω_(diff) between a reference frequency signal ω_(ref) and thegenerator frequency signal, and governor controller 20 is configured forusing the difference in addition to the filtered frequency signal togenerate the power feedback signal P_(fb).

In one example, the governor dynamics (including the prime mover) arerepresented as

${\frac{\mathbb{d}T_{m}}{\mathbb{d}t} = {\frac{1}{T_{G}}( {P_{ref} + \frac{\omega_{ref} - \omega_{m}}{R_{G}} - T_{m}} )}},$wherein T_(M) represents the mechanical torque of the generator, T_(G)represents the time constant of the prime mover, P_(ref) represents thereference power, ω_(ref) represents the reference frequency signal,ω_(m) represents the generator frequency signal of generator 12, R_(G)represents the governor droop, and t represents time.

Bandpass filter 22 typically has a range extending from about 0.1 Hz toabout 100 Hz. In a more specific example the range extends from about 1Hz to about 10 Hz. The filter gain and range are typically selected sothat bandpass filter 22 is configured to increase the speed of detectinga change of the generator frequency signal upon a disruption to theelectrical grid. More specifically, the desired effect of bandpassfilter 22 is to cause the loop formed by frequency sensor 16, bandpassfilter 22, governor summation element 24, prime mover 26, and generator12 to accelerate the effects in any change of the generator frequencysignal.

Even more specifically, bandpass filter 22 typically comprises a filterwith a filter gain comprising a value that is greater than or equal to again required to result in a disconnected grid loop gain of greater thanunity (0 decibels) and a connected grid loop gain of less than unity.The higher the filter gain, the more quickly the generator frequencysignal will move outside the normal operational windows when the grid isdisconnected and the more potential is created for oscillatory dynamicinteractions with the grid when the grid is connected. The filter gaincan be selected to balance the two objectives. One example model forsuch filter gain is as follows:

${\overset{\Delta\omega}{\longrightarrow}K}\;{\frac{{sT}_{w}}{( {1 + {sT}_{w}} )( {1 + {sT}_{1}} )}\overset{\Delta\; p}{\longrightarrow}}$wherein Δω represents ω_(ref) minus ω_(m), K represents the filter gain,T_(w) represents the high pass corner frequency of the bandpass filter,T₁ represents the low pass corner frequency, and Δp represents acorresponding power difference between the resulting power and thereference power. The washout function serves as a high pass filter, witha time constant T_(w) to allow signals with frequencies higher than1/T_(w) to pass. Signals with frequencies lower than 1/T_(w) (especiallyDC signals) will be attenuated, and thus the loop impact is minimizedfor steady-state operation. The low pass filter corner frequency T₁ isset to attenuate high-frequency noise. The selection of filter gain K isa compromise between high enough gain (with some margin) to ensureislanding detection quickly and low enough gain (with some margin) tohave minimal impact on the generator under grid-connected conditions.

The graph of FIG. 4 illustrates a simulation of terminal voltage,frequency, and rate of change of frequency without active anti-islandingcontrol, and the graph of FIG. 5 illustrates simulated parameters withactive power anti-islanding control. As can be seen in these figures,when using the active power anti-islanding control, the frequency (whichstarted at 60 Hz in this example) at time=55 seconds is expected to dropto below 50 Hz as compared with a drop ranging from about 59.6 Hz toabout 59.5 Hz in FIG. 4. Similar larger scale effects are also expectedfrom the change in frequency portions of the graphs.

FIG. 2 is a block diagram of a reactive power anti-islanding protectionsystem 11 in accordance with another embodiment of the present inventionwhich may be implemented either alone or, as shown in FIG. 3, incombination with the embodiment of FIG. 1. In the embodiment of FIG. 2,system 11 comprises a voltage sensor 40 configured to generate aterminal voltage signal V_(t) representative of a terminal voltage ofthe generator, a feedback power calculator 30 configured for generatinga reactive feedback power signal Q_(g), a bandpass filter 32 configuredfor filtering the terminal voltage signal, a proportional-integral (PI)controller summation element 34 configured for summing the filteredterminal voltage signal, the reactive power feedback signal, and areference reactive power Q_(ref) to provide an error signal for use by aproportional integral controller 36 in generating a voltage referencesignal V_(ref) for generator 12.

In a more specific embodiment, anti-islanding protection system 11further comprises grid controller 28 which is configured for receivingthe terminal voltage signal and generating a grid disconnection signalupon the terminal voltage signal becoming out of a nominal terminalvoltage range.

Bandpass filter 32 typically has a range extending from about 0.1 Hz toabout 100 Hz. In a more specific example the range extends from about 1Hz to about 10 Hz. The filter gain and range are typically selected sothat bandpass filter 32 is configured to increase the speed of detectinga change of the terminal voltage upon a disruption to the electricalgrid. More specifically, the desired effect of bandpass filter 32 is tocause the loop formed by terminal voltage sensor 40, bandpass filter 32,feedback power calculator 30, PI controller summation element 34, PIcontroller 36, a voltage regulator summation element 38, an exciter 42,and generator 12 to accelerate the effects in any change of the terminalvoltage signal.

Even more specifically, in a similar manner as discussed with respect tobandpass filter 22 of FIG. 1, bandpass filter 32 typically comprises afilter with a gain comprising a value that is greater than or equal to again required to result in a disconnected grid loop gain of greater thanunity (0 decibels) and a connected grid loop gain of less than unity.The higher the filter gain, the more quickly the terminal voltage willmove outside the normal operational windows when the grid isdisconnected and the more potential is created for oscillatory dynamicinteractions with the grid when the grid is connected. The filter gaincan be selected to balance the two objectives. One example model forsuch filter gain is as follows:

${\overset{\Delta\; V}{\longrightarrow}K}\;{\frac{{sT}_{w}}{( {1 + {sT}_{w}} )( {1 + {sT}_{1}} )}\overset{\Delta\; q}{\longrightarrow}}$wherein ΔV represents V_(ref) minus V_(t), K represents the filter gain,T_(w) represents the high pass corner frequency of the bandpass filter,T₁ represents the low pass corner frequency, and Δq represents Q_(ref)minus Q_(g). The washout function serves as a high pass filter, with atime constant T_(w) to allow signals with frequencies higher than1/T_(w) to pass. Signals with frequencies lower than 1/T_(w) (especiallyDC signals) will be attenuated, and thus the loop impact is minimizedfor steady-state operation. The low pass filter corner frequency T₁ isset to attenuate high-frequency noise. The selection of filter gain K isa compromise between high enough gain (with some margin) to ensureislanding detection quickly and low enough gain (with some margin) tohave minimal impact on the generator under grid-connected conditions.Although it is possible that the filter gain of bandpass filter 32 ofFIG. 2 might be the same as the filter gain of bandpass filter 22 ofFIG. 1, typically the various compromises will result in differentfilter gains for bandpass filters 22 and 32.

Most generators are equipped with an exciter 42 such as an AVR(automatic voltage regulator) type exciter. AVRs maintain the terminalvoltage of the generator at a specific value by reactive (or powerfactor) regulation. In the embodiment of FIG. 2, feedback PI controller36 is cascaded in combination with exciter 42 to regulate the reactivepower of generator 12 so that, when the grid is connected, the reactivepower output of the generator will follow the desired reference value.

As can be seen in comparison to the simulations in the graphs shown inFIG. 4, in the simulations shown in the graphs of FIG. 6, whichillustrate the system with reactive power anti-islanding control, theeffect on voltage measurements dramatically increases with the reactivecontrol. The reactive power anti-islanding control embodimentadditionally appears to have the potential to result in an increasedeffect on the frequency and change of frequency portions of the graphsas well. Therefore, in some embodiments, controller 28 may additionallyor alternatively be configured for receiving a generator frequencysignal representative of the rotating frequency of generator 12 andgenerating a grid disconnection signal upon the generator frequencysignal becoming out of a nominal frequency signal range.

FIG. 3 is a block diagram of a combined active and reactive poweranti-islanding protection system in accordance with another embodimentof the present invention. In the embodiment of FIG. 3, the overallsystem is a combination of systems 10 and 11 and includes a frequencysensor 16 configured to generate a generator frequency signal ω_(m)representative of a rotational speed of the generator, an active loopbandpass filter 32 configured for filtering the generator frequencysignal, a governor controller 20 configured for using the filteredfrequency signal to generate a power feedback signal P_(fb), a governorsummation element 24 configured for summing the filtered frequencysignal, the power feedback signal, and a reference power P_(ref) toprovide an electrical torque signal T_(e) for use by a prime mover 26coupled to the generator, a voltage sensor 40 configured to generate aterminal voltage signal V_(t) representative of a terminal voltage ofthe generator, a feedback power calculator 30 configured for generatinga reactive feedback power signal Q_(g), a reactive loop bandpass filter32 configured for filtering the terminal voltage signal, aproportional-integral controller summation element 34 configured forsumming the filtered terminal voltage signal, the reactive powerfeedback signal, and a reference reactive power Q_(ref) to provide anerror signal for use by a proportional integral controller in generatinga voltage reference signal V_(ref) for the generator.

Each of the above FIG. 1 and FIG. 2 embodiment variations is alsoapplicable in the embodiment of FIG. 3, and an optional common gridcontroller can be used. In this example, grid controller 28 isconfigured for receiving the generator frequency signal and generating agrid disconnection signal upon the generator frequency signal becomingout of a nominal frequency range, and the grid controller is furtherconfigured for receiving the terminal voltage signal and generating agrid disconnection signal upon the terminal voltage signal becoming outof a nominal terminal voltage range.

The previously described embodiments of the present invention have manyadvantages, including the ability to more quickly assess an unintendeddisconnection of a utility power source from a grid due to the positivefeedback which is introduced to destabilize the frequency and/or voltagewhile having minimal effect in the presence of the grid.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A system for providing anti-islanding protection of a distributedgenerator with respect to a feeder connected to an electrical grid, thesystem comprising: a voltage sensor configured to generate a terminalvoltage signal representative of a terminal voltage of the generator; afeedback power calculator configured for generating a reactive feedbackpower signal; a bandpass filter configured for filtering the terminalvoltage signal; a proportional-integral controller summation elementconfigured for summing the filtered terminal voltage signal, thereactive power feedback signal, and a reference reactive power toprovide an error signal for use by a proportional integral controller ingenerating a voltage reference signal for the generator.
 2. The systemof claim 1 further comprising a grid controller configured for receivingthe terminal voltage signal and generating a grid disconnection signalupon the terminal voltage signal becoming out of a nominal terminalvoltage range.
 3. The system of claim 2 wherein the bandpass filtercomprises a filter gain comprising a value that is greater than or equalto a gain required to result in a disconnected grid loop gain of greaterthan unity.
 4. The system of claim 3 wherein a range of the bandpassfilter extends from about 0.1 Hz to about 100 Hz.
 5. The system of claim2 wherein the bandpass filter is configured to increase the speed ofdetecting a change of the terminal voltage signal upon a disruption tothe electrical grid.
 6. The system of claim 2 wherein the gridcontroller is further configured for receiving a generator frequencysignal representative of the rotational speed of the generator andgenerating a grid disconnection signal upon the generator frequencysignal becoming out of a nominal frequency range.
 7. The system of claim1 further comprising a grid controller configured for receiving agenerator frequency signal representative of the rotating frequency ofthe generator and generating a grid disconnection signal upon thegenerator frequency signal becoming out of a nominal frequency range.